金纳米棒的超快电子—声子耦合动力学研究
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摘要
本论文采用飞秒泵浦-探测系统结合去偏振探测法对金纳米棒被光激发后其超快电子-声子耦合动力学过程展开了研究。目的在于探讨金纳米棒作为一种各向异性的贵金属纳米材料,与各向同性的金纳米球相比,其超快电子-声子耦合动力学过程是否会呈现出特殊的性质,以此对其超快电子动力学过程进行深一步地理解和探讨。
主要工作包括:首先,搭建了一套高灵敏度飞秒泵浦-探测(瞬态吸收光谱)系统,主要由掺钛蓝宝石(Ti:Sapphire)飞秒脉冲激光器,泵浦-探测光路,光谱仪(Avantes)和计算机(PC)组成。其次,基于所搭系统采用飞秒瞬态去偏振测试方法对金纳米棒的超快电子-声子耦合动力学过程进行测试表征。最后,基于实验结果分析讨论金纳米棒中的超快电子-声子耦合动力学过程,并提出理论模型进一步验证了实验结果。
实验结果和理论分析表明金纳米棒存在各向异性的电子-声子耦合动力学过程,但它不是一个本征过程,而是在不同的探测光偏振方向上由于金纳米棒的各向异性特性而被分配了不同的激发能分量。由瞬态实验测得的电子-声子耦合动力学不能简单的用单指数函数来描述,而是水溶液中随机分布的各个方向的金纳米棒在探测方向上的分量共同积分作用的结果。在实验使用的相对较高的激发能量下,金纳米棒的电子-声子弛豫时间与激发能量之间是线性关系,其本征电子-声子弛豫时间与金纳米球和体金材料的基本一致。
Considerable work has been done on metal nanoparticles recently because of their many interesting properties like enhanced near electromagnetic fields, wavelength tunable light absorption, and strong photothermal effect as well as potential technological applications including biological sensing, imaging, and nanoelectronic. One extensively studied system is the Au nanoparticle. For Au nanodots, the SPR absorption spectrum contains an intense absorption band in the visible region, while the SPR of Au nanorods splits into two modes: a longitudinal mode parallel to the long axis of the nanorod and a transverse mode perpendicular to it. The absorption peak of the transverse mode is essentially the same with SPR absorption of nanodots, while the longitudinal mode absorbs at lower energy and can be tuned by the aspect ratio. In recent years, there has been a great deal of interest in investigating the electron dynamics in Au nanoparticles after excitation with femtosecond laser pulses. Because of the anisotropy of Au nanorods with two perpendicular resonant modes, Au nanorods have special properties and application relative to the isotropic nanodots, and study of its ultrafast electron dynamics is always a research focus.
In this thesis, the ultrafast electron-phonon (e-ph) coupling dynamics in Au nanorods are studied with femtosecond transient depolarization experiments. The main purpose is to explore whether the ultrafast e-ph coupling dynamics of Au nanorods will present special properties or not relative to that of Au nanodots and bulk Au. The main contents include following three parts:
Firstly, a high-sensitivity femtosecond pump-probe (transient absorption spectroscopy) system has been designed, which consists of Ti:sapphire femtosecond pulse laser, pump-probe part, spectrometer (Avantes) and computer. After optimization of the system by improving the stability of femtosecond laser and the continuum white light, the signal to noise ratio (SNR) of the system is 10-4.
Secondly, the ultrafast electron-phonon (e-ph) coupling dynamics in Au nanorods of two perpendicular probe polarization directions are studied with femtosecond transient depolarization experiments. For one thing, the transient spectra are strongly dependent on the probe polarization directions at all the pump energies. The bleach signal of the longitudinal mode has a larger magnitude when the polarization of the probe beam is parallel to that of the excitation beam. The bleach signals of the transverse mode at two probe directions show reversed trend. In the same probe polarization direction, the transient spectra are also strongly dependent on the pump energy, with larger magnitude at higher pump energy. For another thing, an unexpected anisotropic e-ph coupling dynamics for two probe directions, which differ not only in the magnitudes but also in the relaxation times. When probing at the perpendicular direction, the relaxation time is always short at the same pump power. Additionally, the e-ph relaxation time constants increase with the pump energy in both polarization directions. At low pump power, previous experimental results have indicated that the e-ph relaxation times are proportional to the pump power in Au nanodots and also in nanorods.
Thirdly, analysis and discussion of the ultrafast e-ph coupling dynamics in Au nanorods based on the experimental results, and a theoretical model proposed to further prove our results. Actually, Au nanorod has strong anisotropic character: its longitudinal mode is parallel to the long axis of the nanorod. The effective light intensity to excite the longitudinal mode is the projection of pump intensity to the long axis of Au nanorod. If the polarization direction of probe light has been changed, it will result in the different weights of pump energy assigned to two directions. This may be the reason for the unexpected anisotropic e-ph coupling dynamics. And then we proposed a simple theoretical model to further discuss these anisotropic e-ph coupling dynamics and also pump power dependent relaxation times by fitting the e-ph coupling dynamics in Au nanorods.
Above all, the experimental results and theoretical analysis show that anisotropic e-ph coupling dynamics exists in Au nanorods, which differs not only in the magnitudes but also in the relaxation times. However, this anisotropic e-ph coupling dynamics is not due to an intrinsic process but is the result of different weights of pump energy assigned to two directions because of the anisotropy of Au nanorods. The e-ph coupling kinetics measured by transient experiments cannot be simply described by a single exponential function but by the integral of components contributed by random distribution of Au nanorods at all directions in aqueous solution. The relationship between electron-phonon relaxation times and pump power is linear even in relatively high power. The intrinsic electron-phonon relaxation time is 0.75 ps which is similar to the characteristic electron-phonon coupling time for nanodots and bulk Au.
主要工作包括:首先,搭建了一套高灵敏度飞秒泵浦-探测(瞬态吸收光谱)系统,主要由掺钛蓝宝石(Ti:Sapphire)飞秒脉冲激光器,泵浦-探测光路,光谱仪(Avantes)和计算机(PC)组成。其次,基于所搭系统采用飞秒瞬态去偏振测试方法对金纳米棒的超快电子-声子耦合动力学过程进行测试表征。最后,基于实验结果分析讨论金纳米棒中的超快电子-声子耦合动力学过程,并提出理论模型进一步验证了实验结果。
实验结果和理论分析表明金纳米棒存在各向异性的电子-声子耦合动力学过程,但它不是一个本征过程,而是在不同的探测光偏振方向上由于金纳米棒的各向异性特性而被分配了不同的激发能分量。由瞬态实验测得的电子-声子耦合动力学不能简单的用单指数函数来描述,而是水溶液中随机分布的各个方向的金纳米棒在探测方向上的分量共同积分作用的结果。在实验使用的相对较高的激发能量下,金纳米棒的电子-声子弛豫时间与激发能量之间是线性关系,其本征电子-声子弛豫时间与金纳米球和体金材料的基本一致。
Considerable work has been done on metal nanoparticles recently because of their many interesting properties like enhanced near electromagnetic fields, wavelength tunable light absorption, and strong photothermal effect as well as potential technological applications including biological sensing, imaging, and nanoelectronic. One extensively studied system is the Au nanoparticle. For Au nanodots, the SPR absorption spectrum contains an intense absorption band in the visible region, while the SPR of Au nanorods splits into two modes: a longitudinal mode parallel to the long axis of the nanorod and a transverse mode perpendicular to it. The absorption peak of the transverse mode is essentially the same with SPR absorption of nanodots, while the longitudinal mode absorbs at lower energy and can be tuned by the aspect ratio. In recent years, there has been a great deal of interest in investigating the electron dynamics in Au nanoparticles after excitation with femtosecond laser pulses. Because of the anisotropy of Au nanorods with two perpendicular resonant modes, Au nanorods have special properties and application relative to the isotropic nanodots, and study of its ultrafast electron dynamics is always a research focus.
In this thesis, the ultrafast electron-phonon (e-ph) coupling dynamics in Au nanorods are studied with femtosecond transient depolarization experiments. The main purpose is to explore whether the ultrafast e-ph coupling dynamics of Au nanorods will present special properties or not relative to that of Au nanodots and bulk Au. The main contents include following three parts:
Firstly, a high-sensitivity femtosecond pump-probe (transient absorption spectroscopy) system has been designed, which consists of Ti:sapphire femtosecond pulse laser, pump-probe part, spectrometer (Avantes) and computer. After optimization of the system by improving the stability of femtosecond laser and the continuum white light, the signal to noise ratio (SNR) of the system is 10-4.
Secondly, the ultrafast electron-phonon (e-ph) coupling dynamics in Au nanorods of two perpendicular probe polarization directions are studied with femtosecond transient depolarization experiments. For one thing, the transient spectra are strongly dependent on the probe polarization directions at all the pump energies. The bleach signal of the longitudinal mode has a larger magnitude when the polarization of the probe beam is parallel to that of the excitation beam. The bleach signals of the transverse mode at two probe directions show reversed trend. In the same probe polarization direction, the transient spectra are also strongly dependent on the pump energy, with larger magnitude at higher pump energy. For another thing, an unexpected anisotropic e-ph coupling dynamics for two probe directions, which differ not only in the magnitudes but also in the relaxation times. When probing at the perpendicular direction, the relaxation time is always short at the same pump power. Additionally, the e-ph relaxation time constants increase with the pump energy in both polarization directions. At low pump power, previous experimental results have indicated that the e-ph relaxation times are proportional to the pump power in Au nanodots and also in nanorods.
Thirdly, analysis and discussion of the ultrafast e-ph coupling dynamics in Au nanorods based on the experimental results, and a theoretical model proposed to further prove our results. Actually, Au nanorod has strong anisotropic character: its longitudinal mode is parallel to the long axis of the nanorod. The effective light intensity to excite the longitudinal mode is the projection of pump intensity to the long axis of Au nanorod. If the polarization direction of probe light has been changed, it will result in the different weights of pump energy assigned to two directions. This may be the reason for the unexpected anisotropic e-ph coupling dynamics. And then we proposed a simple theoretical model to further discuss these anisotropic e-ph coupling dynamics and also pump power dependent relaxation times by fitting the e-ph coupling dynamics in Au nanorods.
Above all, the experimental results and theoretical analysis show that anisotropic e-ph coupling dynamics exists in Au nanorods, which differs not only in the magnitudes but also in the relaxation times. However, this anisotropic e-ph coupling dynamics is not due to an intrinsic process but is the result of different weights of pump energy assigned to two directions because of the anisotropy of Au nanorods. The e-ph coupling kinetics measured by transient experiments cannot be simply described by a single exponential function but by the integral of components contributed by random distribution of Au nanorods at all directions in aqueous solution. The relationship between electron-phonon relaxation times and pump power is linear even in relatively high power. The intrinsic electron-phonon relaxation time is 0.75 ps which is similar to the characteristic electron-phonon coupling time for nanodots and bulk Au.
引文
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[2] Goodberlet J, Wang J, Fujimoto J G, Schulz P A. Starting dynamics of additive pulse mode locking in the Ti: A12 03 laser [J].Opt . Lett.,1990,15: 1300-1302.
[3] Schenkel B, Biegert J, Keller U, Vozzi C, Nosoli M, Sansone G, Stagira S, De-Silvestri S and Svelto O. Generation of 3.8-fs pulses from Adaptive compression of a cascaded hollow fiber supercontinuum [J].Opt. Lett.,2003,28:1987-1989.
[4] Huang X H, Neretina S, El-Sayed M A. Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications [J].Adv. Mater.,2009,21:4880-4910.
[5] Link S, El-Sayed M A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods [J].J. Phys. Chem. B,1999,103:8410-8426.
[6] Link S, El-Sayed M A. OPTICAL PROPERTIES AND ULTRAFAST DYNAMICS OF METALLIC NANOCRYSTALS [J].Annu. ReV. Phys. Chem.,2003,54:331-366.
[7] Hartland G V. Coherent excitation of vibrational modes in metallic nanoparticles [J].Annu. ReV. Phys. Chem.,2006,57:403-430.
[8] Varnavski O P, Goodson T, Mohamed M B, El-Sayed M A. Femtosecond excitation dynamics in gold nanospheres and nanorods [J]. Phys. ReV. B,2005,72:2354051-2354059.
[9] Huang W Y, Qian W, El-Sayed M A, Ding Y, Wang Z L. Effect of the Lattice Crystallinity on the Electron?Phonon Relaxation Rates in Gold Nanoparticles [J].J. Phys. Chem. C,2007,111(29),10751-10757.
[10]Ramakrishna G, Dai Q, Zou J H, Huo Q, Goodson T III. Interparticle electromagnetic coupling in assembled gold-necklace nanoparticles [J].J. Am. Chem. Soc.,2007,129:1848-1849.
[11]Park S, Pelton M, Liu M, Guyot-Sionnest P, Scherer N F. Ultrafast Resonant Dynamics of Surface Plasmons in Gold Nanorods [J].J. Phys. Chem. C,2007,111:116-123.
[12]Pelton M, Liu M, Park S, Scherer N F, Guyot-Sionnest P. Ultrafast resonant optical scattering from single gold nanorods: Large nonlinearities and plasmon saturation [J].Phys. ReV. B 2006, 73, 155419-155424.
[13]Hartland G V. Measurements of the material properties of metal nanoparticles by time-resolved spectroscopy [J].Phys. Chem. Chem. Phys.,2004,6:5263-5274.
[14]Voisin C, Christofilos D, Loukakos P A, Del Fatti N, Valle′e F. Ultrafast electron-electron scattering and energy exchanges in noble-metal nanoparticles [J].Phys. ReV. B,2004,69:195416-195428.
[15]Voisin C, Del Fatti N, Christofilos D, Valle′e F. Ultrafast Electron Dynamics and Optical Nonlinearities in Metal Nanoparticles [J].J. Phys. Chem. B,2001,105:2264-2280.
[16]Grant C D, Schwartzberg A M, Norman T J, Jr, Zhang J Z. Ultrafast Electronic Relaxation and Coherent Vibrational Oscillation of Strongly Coupled Gold Nanoparticle Aggregates [J].J. Am. Chem. Soc.,2003,125:549-553.
[17]Link S, Burda C, Mohamed M B, Nikoobakht B, El-Sayed M A. Femtosecond transient-absorption dynamics of colloidal gold nanorods: Shape independence of the electron-phonon relaxation time [J].Phys. ReV. B,2000,61:6086-6090.
[18]Chen S W, Pei R J. Ion-Induced Rectification of Nanoparticle Quantized Capacitance Charging in Aqueous Solutions [J].J. Am. Chem. Soc.,2001,123:10607-10615.
[19]Henglein A, Giersig M. Optical and Chemical Observations on Gold? Mercury Nanoparticles in Aqueous Solution [J].J. Phys. Chem. B,2000,104:5056-5060.
[20]Henglein A. Colloidal Palladium Nanoparticles: Reduction of Pd(II) by H2; PdCoreAuShellAgShell Particles [J].J. Phys. Chem. B,2000,104:6683-6685.
[21]Park S, Yang P X, Corredor P, Weaver M J. Transition Metal-Coated Nanoparticle Films: Vibrational Characterization with Surface-Enhanced Raman Scattering [J].J. Am. Chem. Soc.,2002,124:2428-2429.
[22]Park J I, Cheon J. Synthesis of“Solid Solution”and“Core-Shell”Type Cobalt ? Platinum Magnetic Nanoparticles via Transmetalation Reactions [J].J. Am. Chem. Soc.,2001,123:5743-5746.
[23]Krug J T, Wang G D, Emory S R, Nie S M. Efficient Raman Enhancement and Intermittent Light Emission Observed in Single Gold Nanocrystals [J].J. Am. Chem.Soc.,1999,121:9208-9214.
[24]Averitt R D, Westcott S L, Halas N J. Ultrafast electron dynamics in gold nanoshells [J].Phys. ReV. B,1998,58:10203-10206.
[25]Templeton A C, Hostetler M J, Warmoth E K, Chen S W, Hartshorn C M, Krishnamurthy V M, Forbes M D E, Murray R W. Gateway Reactions to Diverse, Polyfunctional Monolayer-Protected Gold Clusters [J].J. Am. Chem. Soc.,1998,120:4845-4849.
[26]Kreibig U, Vollmer M. Optical properties of metal clusters [M].Berlin and New York:Springer,1995.
[27]Kerker M. The Scattering of Light and Other Electromagnetic Radiation [M]. New York: Academic,1969.
[28]Bohren C F, Huffman D R. Absorption and Scattering of Light by Small Particles [M].New York:John Wiley & Sons,1983.
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